This invention relates generally to data transmission systems, and more particularly the invention relates to a signal modulator for data transmitted in a cable transmission system.
Much attention is being directed to converting unidirectional analog data transmission systems, such as the community antenna television (CATV) cable system into more versatile bidirectional communication systems. Today, over 60 million households in the United States enjoy the benefits of cable TV, virtually all of the information which travels into the home over the cable is in the form of analog television signals. Some subscribers now have the ability to send digital commands to select movies or provide other forms of low rate data information from the home to a central location. However, in the next few years the rate of digital information both entering and leaving the home over the CATV cable will increase dramatically.
Capabilities now being envisioned to exploit the band width of the cable include all conventional telephony services and a variety of multimedia capabilities including digital video or high speed "video phone", remote video games, high speed data and a wide variety of consumer purchasing and voting services. As the computer is merged with the cable, the consumer will be able to rent movies, play games, take college courses, make travel plans, trade stocks, and the like.
Traditionally, cable data transmission systems provide one-way transmission, from head-end to subscriber, or the forward channel in a frequency spectrum up to 750 MHz. Typically, the spectrum from 54-550 MHz is dedicated to analog video for the forward channel, 550-700 MHz is dedicated to digital video, and 700-750 MHz is dedicated to voice and multimedia in the forward channel. To satisfy the requirement for interactivity in the information superhighway, a return path has to be provided. Cable operators have allocated a frequency spectrum of 5-40 MHz for this reverse channel.
The present invention is directed to apparatus for providing a modulated tunable signal by direct frequency synthesis directly to the cable medium, particularly from a subscriber to the head end unit in the reverse channel, without the use of an upconverter.